Autocomplete action inputs

**Describe the solution you'd like**
I would like to have auto completion for action inputs, so `ctrl + space`ing will show the possible inputs, and an error when a required input is not set, or when an invalid input is being passed

The issue does not identify files, tests, or an entry point. Start by locating the extension's action-input completion and validation code, then determine how action metadata is represented. Done means Ctrl+Space suggests valid inputs and the editor reports missing required or invalid inputs.

Written by the indexing model from the issue text.